Nowadays, with the advancement of miniaturization techniques, it is possible to manufacture ultrafine and ultrafast semiconductor devices having a gate length of less than 0.1 μm.
In those ultrafine and ultrafast semiconductor devices, as a gate length is reduced, a film thickness of a gate oxide film needs to be reduced in accordance with the scaling law. In the semiconductor devices having a gate length of less than 0.1 μm, when a conventional thermally-oxidized film is used, the film thickness of the gate oxide film needs to be set from 1 to 2 nm or less. However, in such a very thin gate insulator, tunneling current is increased, so that a problem of increasing a gate leakage current is inevitable as a result.
In view of this, conventionally, there has been proposed a high dielectric material (so called a high-K film) such as Ta2O5, Al2O3, ZrO2, HfO2, ZrSiO4, and HfSiO4 applied to the gate insulator, in which a dielectric constant thereof is substantially greater then that of the thermally-oxidized film and thus an equivalent SiO2 film thickness is small even though an actual film thickness is large. By using such a high dielectric material, it is possible to use a gate insulator having a physical film thickness of several nanometers even in an ultrafast semiconductor device having a very short gate length of not more than 0.1 μm and to reduce the gate leakage current resulting from the tunneling effect. In general, when such a high dielectric material is formed on a surface of a silicon substrate, the material has a polycrystalline structure.
When a high dielectric film is directly formed on the surface of the silicon substrate, a large-scale interdiffusion of Si atoms and metallic atoms is likely to be generated between the silicon substrate and the high dielectric film. In accordance with this, in general, the high dielectric film is formed on the surface of the silicon substrate via a very thin interfacial oxide film. On the other hand, there has been proposed a technique of directly forming the high dielectric film on the surface of the silicon substrate by selecting a material of the high dielectric film in recent years.